The present invention relates to a gas diffusion electrode suitable for production of chlorine and alkali metal hydroxide. The invention also concerns a method for manufacturing such a gas diffusion electrode. The invention further concerns an electrolytic cell comprising such gas diffusion electrode and the use thereof.
Electrolysis of alkali metal chlorides to produce chlorine and alkali metal hydroxide has been known for a long time.
In the past, hydrogen evolving cathodes have been used for this purpose. The principal chemical reaction taking place in the electrolytic cell can be represented by the following scheme: 2NaCl+2H2Oxe2x86x92Cl2+2 NaOH+H2. This electrolysis reaction, having a theoretical cell voltage of 2.24 V, requires a considerable amount of energy.
Previously, also oxygen consuming gas diffusion electrodes have been disclosed for the production of chlorine and alkali metal hydroxide, as further described in e.g. U.S. Pat. No. 4,578,159. The term xe2x80x9cgas diffusion electrodexe2x80x9d, as used herein, relates to an electrode, comprising a hydrophobic gas diffusion layer and a reaction layer, and suitably an electrode substrate, to which gas diffusion electrode oxygen-containing reactant gas is supplied to undergo electrolysis. Electrolyte is supplied to one area of the electrode, different from the area to which reactant gas is supplied. The principal reaction taking place at the reaction layer of the electrode may be represented by the following reaction scheme: 2NaCl+H2O+xc2xdO2xe2x86x92Cl2+2NaOH, the theoretical cell voltage being 0.96 V, i.e. only about 40% of the cell voltage of the hydrogen evolving electrode. Therefore, the gas diffusion electrode considerably reduces the energy costs of the operation of the electrolytic cell.
In previously employed partitioned electrolytic cell arrangements, wherein gas diffusion electrodes have been directly contacted to an ion exchange membrane, dividing the electrolytic cell into a cathode compartment and an anode compartment, electrolyte flooding problems have been faced due to the fact that the diffusion of oxygen-containing gas supplied to the gas diffusion electrode has been impeded by electrolyte present in the cathode compartment. This problem can, however, be overcome by arranging a hydrophilic layer between the reaction layer and the ion exchange membrane, thereby providing a flood-preventing gap in between.
In this type of electrode arrangements, however, it has been noticed that the catalytic material present in the reaction layer of the electrode in contact with the hydrophilic layer, undesirably catalyses an oxidation reaction of the hydrophilic layer, usually comprising carbon, which causes formation of carbonates. Carbonates, in turn, undesirably increase the hydrophilicity of the hydrophobic gas diffusion layer, leading to a decreased diffusion of supplied gas to the reaction layer of the electrode. This fact results in an increase of the cell voltage and destabilises the operation of the electrolytic cell.
The present invention intends to solve the above problems.
The present invention relates to a gas diffusion electrode comprising a hydrophobic gas diffusion layer, a reaction layer, a barrier layer, and a hydrophilic layer, arranged in the mentioned order.
It has been surprisingly found that the problems referred to above concerning unwanted catalytic oxidation of the material in the hydrophilic layer can be solved by providing a barrier layer between the hydrophilic and reaction layers. The barrier layer thus provides a barrier preventing unwanted oxidation processes to occur by impeding contact between the two layers. The barrier layer also secures stable operation of the electrolytic cell, in which the gas diffusion electrode is arranged, thereby preventing any substantial fluctuation in cell voltage or current density. Moreover, it has been found that the inventional gas diffusion electrode can be operated substantially without any other deteriorating effects. The barrier layer further provides good adhesion to its adjacent layers.
According to one preferred embodiment, the hydrophobic gas diffusion layer is arranged to one side of an electrode substrate. The electrode substrate is further, on its opposite side, suitably arranged to the reaction layer.
The hydrophobic gas diffusion layer is suitably made of silver, or silver-plated metals, e.g. silver-plated nickel, and hydrocarbon polymers such as vinyl resins, polyethylene, polypropylene, or other hydrocarbon polymers; halocarbon polymers containing chlorine, fluorine, or both, including fluoropolymers such as polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP), polychlorofluoroethylene or mixtures thereof, preferably PTFE. The polymers suitably have a molecular weight of 10,000 g/mole or more.
The reaction layer suitably comprises at least one catalytically active material for the production of alkali metal hydroxide. The material may include silver, platinum, platinum group metals, or mixtures thereof, preferably platinum, silver or mixtures thereof. Also a polymeric binder may be included in the reaction layer such as polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP), fluoro polymers such as nafion(trademark) (perfluorocarbon sulfonic acid resin) and derivatives thereof, or other halocarbon polymers such as polychlorofluoroethylene or mixtures thereof, preferably polytetrafluoroethylene (PTFE) or nafion(trademark) or mixture or derivatives thereof.
According to one preferred embodiment, the reaction layer is arranged to the electrode substrate on the opposite side of the hydrophobic gas diffusion layer. The electrode substrate is suitably made of a conductive expanded metal, a mesh or the like. The substrate material may be silver or silver plated metals such as silver plated stainless steel, silver plated nickel, silver plated copper, gold, gold plated metals such as gold plated nickel, or gold plated copper; nickel, cobalt, cobalt plated metals such as cobalt plated copper, or mixtures thereof, preferably silver or silver plated metals. Polymers such as halocarbon polymers can also be incorporated in the electrode substrate as very finely divided particulate solids, e.g. micron-sized particles.
By barrier layer, is meant to include any layer comprising a material functioning as a layer separating the hydrophilic and reaction layers, thereby preventing contact between the hydrophilic and the reaction layer, especially to impede the catalyst particles in the reaction layer to catalyse the oxidation of carbon present in the hydrophilic layer to form carbonates. The barrier layer suitably is substantially made of a ceramic material such as zirconium oxides, e.g. zirconia (ZrO2), titanium oxides, e.g. TiO2, Ti4O7, and hafnium oxides, e.g. HfO2, or mixtures thereof, preferably of zirconia (ZrO2) or mixtures thereof. Further suitable barrier materials include other materials resistant to alkaline environment, such as SiC, BN, TiN, SiO2. Binder material such as PTFE or nafion(trademark) or the like may also be mixed with ceramic or barrier materials to form a barrier layer, suitably forming a barrier layer comprising less than 30 wt % binder material.
The hydrophilic layer is suitably a porous material resistant to electrolytes present in the cathode compartment e.g. alkaline solutions such as caustic soda or the like. Suitably, the hydrophilic layer comprises carbon such as carbon cloth, porous carbon, sintered carbon, or mixtures thereof. The hydrophilic layer is suitably, on the opposite side of the barrier layer, arranged to a separator partitioning an electrolytic cell into a cathode compartment, containing the gas diffusion electrode, and an anode compartment.
According to one preferred embodiment, the layers of the gas diffusion electrode of the invention are arranged to one another by means of coating.
According to a further preferred embodiment, the inventional gas diffusion electrode comprises an electrode substrate made of a silver mesh substrate, a silver paste mixture comprising silver powder and PTFE sintered to the substrate, a reaction layer arranged to one side thereof comprising a silver and/or platinum layer, on which reaction layer is deposited a barrier layer of 70 wt % ZrO2 powder mixed with a 30 wt % PTFE, nafion(trademark), or mixtures thereof to which barrier layer a hydrophilic layer is arranged. A conventional hydrophobic gas diffusion layer is arranged to the opposite side of the reaction layer.
Any other embodiment of a gas diffusion electrode, suitably an oxygen depolarised gas diffusion electrode, provided with a barrier layer as above described also is part of the this invention, e.g. semihydrophobic, liquid or gas permeable gas diffusion electrodes.
The invention also relates to a method for manufacturing a gas diffusion electrode comprising arranging a hydrophobic gas diffusion layer, a reaction layer, a barrier layer and a hydrophilic layer to each other in the mentioned order.
The layers of the gas diffusion electrode are preferably arranged one to the other by means of coating.
According to one preferred embodiment, the method comprises arranging the hydrophobic gas diffusion layer to one side of an electrode substrate, and arranging the reaction layer to the opposite side of said electrode substrate. Preferably, the hydrophobic gas diffusion layer and the reaction layer are arranged to the electrode substrate by means of coating.
According to yet a further preferred embodiment of the invention, the method for manufacturing the gas diffusion electrode comprises:
1) providing a substrate, suitably by spreading a powder paste over a net, paste is subsequently sintered to the net at a temperature of suitably from about 150xc2x0 C. to about 500xc2x0 C., preferably from about 200 to about 240xc2x0 C., thereby providing an electrode substrate;
2) applying an electrocatalytic powder paste and/or solution on one side of the electrode substrate to form a reaction layer, and a gas diffusion hydrophobic layer on the opposite side thereof, and optionally simultaneously applying a binder solution on both sides of the substrate. The electrocatalytic powder paste and/or solution and the optional binder solution is suitably baked at a temperature from about 100 to about 120xc2x0 C.
3) applying a barrier layer to the reaction layer; and
4) arranging a hydrophilic layer to the barrier layer.
Suitably, the powder paste of step 1 is silver powder paste, gold powder paste, or mixtures thereof, preferably silver paste. The net, on which the powder paste is sintered, is suitably made of silver or silver plated metals such as silver plated stainless steel, silver plated nickel, silver plated copper, gold, gold plated metals such as gold plated nickel, gold plated copper; nickel, cobalt, cobalt plated metals such as cobalt plated copper, or mixtures thereof, preferably silver or silver plated metals. The optionally applied binder solution of step 2 suitably is polytetrafluoroethylene (PTFE), fluoro polymers such as nafion(trademark) or derivatives thereof, which suitably comprise perfluorocarbon sulfonic acid type resin, fluorinated ethylene-propylene copolymer (FEP), or other halocarbon polymers such as polychlorofluoroethylene or mixtures thereof, preferably polytetrafluoroethylene (PTFE), preferably nafion(trademark). The applying of an electrocatalytic powder paste and/or solution can also be performed simultaneously with step 1 or 3. To impart good affinity avoiding direct contact between the reaction layer and the hydrophilic layer, the reaction layer is provided with a barrier layer of e.g. ZrO2.
The obtained gas diffusion electrode structure is subsequently arranged to a hydrophilic layer, which hydrophilic layer is suitably directly arranged to a separator partitioning the cathode and anode compartments of an electrolytic cell.
The invention further concerns an electrolytic cell comprising an anode compartment and a cathode compartment partitioned by a separator, wherein an anode is arranged in the anode compartment and the above described gas diffusion electrode is arranged in the cathode compartment. Any suitable anode may be employed in the anode compartment. The gas diffusion electrode may be arranged as plural belt-shaped electrode members or in an electrode patchwork configuration, as further described in U.S. Pat. No. 5,938,901.
The separator, suitably is a commercially available ion exchange membrane, such as Nafion(trademark), preferably a cation exchange membrane, made of a solid polymer electrolyte that transfers ionic charge due to fixed ion exchange groups attached to backbone chains. The membrane used suitably is an inert, flexible membrane, substantially impervious to hydrodynamic flow of the electrolyte and the passage of gas products produced in the cell. The ion exchange membrane may comprise a perfluorinated backbone coated with attached fixed ionic groups such as sulphonic or carboxylic radicals. The terms xe2x80x9csulfonicxe2x80x9d and xe2x80x9ccarboxylicxe2x80x9d are meant to include salts of sulfonic and carboxylic acids which are suitably converted to or from the acid groups by processes such as hydrolysis. Also non-perfluorinated ion exchange membranes or anion exchange membranes comprising quaternary amines on a polymeric support may be used.
The invention also concerns a process for the production of chlorine and alkali metal hydroxide in an electrolytic cell as described above comprising
a) supplying an oxygen-containing gas to the cathode compartment
b) supplying an aqueous solution of alkali metal chloride to the anode compartment
c) passing an electric current through the cell from the anode to the gas diffusion electrode thereby forming alkali hydroxide and in the cathode compartment and chlorine in the anode compartment.